Mixing Character and Meandering Mechanism of a Plane Jet Bounded in a Shallow Water Layer

Abstract: A plane jet in a shallow water layer is characterized by the jet width b1/2 much larger than the water depth h at large downstream distances x/h > 1. Instantaneous field concentration distributions have been measured with a Laser-Induced Fluorescence system to illustrate the role of large scale turbulent structures in the mixing process. Mean and r.m.s. concentration field distributions are obtained by averaging hundreds of digital image frames. It is found that the r.m.s. concentration on the centerline decays as a function of x/h where x is the longitudinal coordinate and h is the water depth. This is different from the usual jet where the flow characters are the function of x/B (B is the jet exit width). By doing instability analyses, critical bottom friction parameters Sc are obtained as a function of the jet co-flow ratio. From the viewpoint of suppression of the jet instability by viscosity or bottom friction, there is a similarity between a low Reynolds number jet in unbounded conditions and a high Reynolds number jet in a shallow fluid layer. This explains the meandering behavior of shallow jets.